1. Filed of the Invention
The present invention relates to a composite film for a condenser, a method for the production thereof and a base film for use therefor. More specifically, it relates to a composite film for a condenser which film comprises, as a base film, a biaxially oriented film containing polyethylene-2,6-naphthalenedicarboxylate as a main polymer component and an electrically conductive metal thin film layer formed thereon, a method for the production of the composite film and a base film for use in the composite film.
2. Prior Art
Having excellent mechanical properties, thermal properties and heat resistance, a biaxially oriented film containing polyethylene-2,6-naphthalenedicarboxylate as a main polymer component is used as a base film (dielectric) for a condenser, and the production thereof is increasing.
Meanwhile, as it is required to decrease electric or electronic circuits in size in recent years, it is also required to decrease the condenser in size and increase the capacitance thereof. It is therefore required to decrease the thickness of a film made of a dielectric as a base. In a film condenser, the thickness of the film as a dielectric is decreased for a reason that the electrostatic capacity of the condenser is (a) in proportion to the dielectric constant and the electrode area of the dielectric and (b) in inverse proportion to the thickness of the film. In other words, the electrostatic capacity of a dielectric per unit volume is in inverse proportion to a square of a film thickness and in proportion to a dielectric constant. If it is attempted to decrease a condenser in size or increase the capacitance thereof while using dielectrics having one dielectric constant, it is inevitable to decrease the film thickness.
It is clear that decreasing the film thickness has the above effect. However, simply decreasing conventional biaxially oriented films in thickness causes new problems. For example, decreasing the film thickness involves poor workability during deposition of electrodes on a film or in the step of slit or winding elements.
The above workability is mainly concerned with the slipperiness of a film. For improving the slipperiness, there is known a method of imparting the film surface with microscopic hills and valleys, and the method is employed. As this method, there is known a method in which inert inorganic fine particles are added to a polyester as a film material during or after polymerization for the polyester (external particle addition method), or part or the whole of a catalyst, etc., used for the polymerization for the polyester is precipitated in a polymer in a reaction step (internal particle precipitation method).
However, when the method for producing a very thin film uses a polymer containing inert inorganic particles having the same concentration as that of inert inorganic particles used for producing a film having an intermediate thickness, the number of the inert inorganic fine particles per unit area of the very thin film decreases, and one fine particle is more widely spaced from another fine particle in the film surface, so that the film surface is flattened and shows a decrease in slipperiness. Further, a very thin film has low stiffness and films adhere easily each other, thereby causing the slipperiness to decrease. For offsetting a decrease in slipperiness caused by a decrease in thickness of the film, therefore, it is required to increase the concentration of the inert inorganic fine particles added to a polymer or increase the size thereof with a decrease in the thickness of the film.
When a polymer containing inert inorganic fine particles is melt-extruded, voids are liable to be formed around the inert inorganic fine particles in the film due to poor affinity between the inert inorganic fine particles and the polymer if a high draft ratio is employed or if the film is further stretched, and the frequency of such formation also increases. Therefore, not only the obtained film is poor in mechanical properties (such as breaking strength or breaking elongation) and electrical properties (such as electrical insulation defect), but also breaking is liable to take place during the production of the film, which causes problems such as a decrease in productivity and an absence of stability of production conditions.
As a film for making it possible to decrease the occurrence of voids, JP-A-1-266145 (JP-B-7-47645) discloses a thin thermoplastic film which is excellent in workability (handling properties) and improved in film formability (non-breaking properties) and has a thickness of 4 xcexcm or less, and which comprises 0.01 to 3% by weight of porous inert inorganic particles having a porosity of 50 to 95% and having an average particle diameter of 0.05 to 5 xcexcm and 0.005 to 1% by weight of spherical silica particles having an average particle diameter of 0.2 to 4 xcexcm which is greater than the thickness of the film in a thermoplastic polymer constituting a film.
Studies made by the present inventors have showed that, when a polyethylene-2,6-naphthalenedicarboxylate film is used as the above film, the film has advantages that it shows no decrease in the mechanical properties, that its heat resistance and electric insulating properties are excellent and that voids do not occur much. However, it has been shown that the film still has a problem that it suffers breaking during the formation thereof and therefore impairs productivity of the film.
A polyethylene-2,6-naphthalenedicarboxylate film generally has a characteristic feature that it has poor tear resistance properties as compared with a polyethylene terephthalate film, and this feature causes the trouble of breaking to take place frequently during the formation of the film. The factor which has not caused any problem during the formation of the polyethylene terephthalate film causes the breaking in the production of the polyethylene-2,6-naphthalenedicarboxylate film in some cases.
JP-A-7-57964 discloses a polyethylene-2,6-naphthalenedicarboxylate film containing two types of porous silica particles having different average particle diameters in a specific mixing ratio. However, this film involves a problem that it has poor productivity since it also breaks frequently during its formation.
It has been also made clear that each of the above polyethylene-2,6-naphthalenedicarboxylate films has a thick air layer due to surface protrusions when stacked one sheet thereof is stacked on another, whereby they are not sufficient in insulating properties and space factor, so that there is still a problem to be solved that excellent workability is not compatible with the space factor and insulating properties.
In the production of a film condenser, further, for avoiding poor productivity caused by decreasing the thickness of a film as a dielectric and poor workability (deposition of a metal for electrodes on a film, slitting and winding of an element) in the step of processing a thin film to a condenser, JP-A-10-294237 proposes that two different types of inert fine particles as a lubricant be added to a polyethylene-2,6-naphthalate film.
The above film is improved in workability in the steps of forming a thin film and processing the film into a condenser. Since, however, an insulating failure is caused due to additives (the lubricants in particular) contained in the film, the above film is not yet satisfactory as a film for a condenser.
Problems to Be Solved by the Invention
It is a first object of the present invention to provide a very thin biaxially oriented film made of polyethylene-2,6-naphthalenedicarboxylate which is for use as a dielectric in a high-quality film condenser having excellent physical and electrical properties.
It is a second object of the present invention to provide a biaxially oriented film having advanced surface properties so that it is suitable for exhibiting processability for a film condenser and electric properties of a film condenser even if it is a very thin biaxially oriented film.
It is a third object of the present invention to provide a biaxially oriented film which has excellent workability in the steps of producing a film condenser (metal deposition, winding of an element, cutting and metallized contact) and has sufficient heat resistance and mechanical strength in these steps.
It is another object of the present invention to provide a composite film for a film condenser excellent in electric properties, dielectric breakdown voltage in particular, and a composite film for a film condenser whose electrical insulation defects are as small as possible.
It is still another object of the present invention to provide a small-sized high-capacitance film condenser.
Means to Solve the Problems
According to studies made by the present inventors, the above objects of the present invention are achieved by the following inventions (I) to (IV).
(I) A composite film for a film condenser comprising a biaxially oriented film made of polyethylene-2,6-naphthalenedicarboxylate as a main polymer component and an electrically conductive metal thin layer formed on the surface of the biaxially oriented film,
wherein the number of flyspecks having an average diameter of 60 xcexcm or more in the surface of said biaxially oriented film is 20/m2 or less and the number of portions failing to satisfy a dielectric breakdown voltage of 200 V/xcexcm (electrical insulation defects) in said composite film is 20/m2 or less.
(II) A method for producing a film condenser member, which comprises depositing an electrically conductive metal on the surface of a biaxially oriented film made of polyethylene-2,6-naphthalenedicarboxylate as a main polymer component to obtain a composite film having an electrically conductive metal thin film layer on the surface thereof and laminating a plurality of the composite films,
wherein, as said biaxially oriented film, a film satisfying the following requirements (1) to (3) is used,
(1) the film has a central line average surface roughness (Ra), in the surface thereof, of 40 to 80 nm,
(2) the number of flyspecks having an average diameter of 60 xcexcm or more in the surface thereof is 20/m2 or less, and
(3) after the film is heat-treated at 200xc2x0 C. for 10 minutes, the film has a thermal shrinkage factor in the longitudinal direction (MD) and a thermal shrinkage factor in the transverse direction (TD) in a thermal shrinkage factor ratio (MD/TD) of 0.39 to 0.82.
(III) A film condenser composed of a laminated composite material formed of a plurality of composite films each of which comprises a biaxially oriented film made of polyethylene-2,6-naphthalenedicarboxylate as a main polymer component and an electrically conductive metal thin layer formed on the surface of the biaxially oriented film,
wherein the number of flyspecks having an average diameter of 60 xcexcmor more in the surface of said biaxially oriented film is 20/m2 or less and the percentage of a failure to satisfy a dielectric breakdown voltage of 200 V/xcexcm (electrical insulation defects) in the film condenser is 10% or less.
(IV) A polyester film for a condenser, which is a biaxially oriented film made of polyethylene-2,6-naphthalenedicarboxylate as a main polymer component, the film satisfying the following requirements (1) to (3),
(1) the film has a central line average surface roughness (Ra), in the surface thereof, of 40 to 80 nm,
(2) the number of flyspecks having an average diameter of 60 xcexcm or more in the surface thereof is 20/m2 or less, and
(3) after the film is heat-treated at 200xc2x0 C. for 10 minutes, the film has a thermal shrinkage factor in the longitudinal direction (MD) and a thermal shrinkage factor in the transverse direction (TD) in a thermal shrinkage factor ratio (MD/TD) of 0.39 to 0.82.
The present invention will be explained further in detail hereinafter.
First, the very thin biaxially oriented film as a dielectric, used in the composite film for a film condenser and the production method thereof provided by the present invention, will be explained.
The above biaxially oriented film will be sometimes abbreviated as xe2x80x9cbase filmxe2x80x9d in the present invention. The base film is made of polyethylene-2,6-naphthalenedicarboxylate as a main polymer component.
The polyethylene-2,6-naphthalenedicarboxylate is a polyester made from 2,6-naphthalenedicarboxylic acid as a main dicarboxylic acid component and ethylene glycol as a main glycol component. The above term xe2x80x9cmainxe2x80x9d means that at least 90 mol %, preferably at least 95 mol % of all of recurring units constituting the polyester is ethylene-2,6-naphthalenedicarboxylate units.
The polyethylene-2,6-naphthalenedicarboxylate in the present invention may be a copolyester. The copolyester is preferably a copolyester which does not impair the inherent properties of a polyethylene-2,6-naphthalenedicarboxylate (hompolymer) film and which secures insulating properties, mechanical properties and thermal dimensional stability. Further, it is preferably a copolyester formable into a film excellent in anti-breaking properties and slitting properties.
When a copolyester is employed, a comononer other than the 2,6-naphthalenedicarboxylic acid and ethylene glycol components can be selected from compounds having a molecule containing two ester-forming functional groups. Examples of such compounds include dicarboxylic acids such as oxalic acid, adipic acid, phthalic acid, sebacic acid, dodecanedicarboxylic acid, isophthalic acid, terephthalic acid, 1,4-cyclohexanedicarboxylic acid, 4,4xe2x80x2-diphenyldicarboxylic acid, phenylindanedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, tetralindicarboxylic acid, decalindicarboxylic acid and diphenyl ether dicarboxylic acid; hydroxycarboxylic acids such as p-hydroxybenzoic acid and p-hydroxyethoxybenzoic acid; and dihydric alcohols such as propylene glycol, trimethylene glycol, tetramethylene glycol, hexamethylene glycol, cyclohexanedimethylene glycol, neopentyl glycol, an ethylene oxide adduct of bisphenol sulfone, an ethylene oxide adduct of bisphenol A, diethylene glycol and polyethylene oxide glycol.
The above comonomers may be used alone or may be used in combination of at least two at the same time. Of these, isophthalic acid, terephthalic acid, 4,4xe2x80x2-diphenyldicarboxylic acid, 2,7-naphthalenedicarboxylic acid and p-hydroxybenzoic acid are more preferred as an acid component, and trimethylene glycol, hexamethylene glycol, neopentyl glycol and an ethylene oxide adduct of bisphenol sulfone are more preferred as a glycol component.
In the polyethylene-2,6-naphthalenedicarboxylate, further, part or all of terminal hydroxyl group and/or carboxyl group may be blocked with a monofunctional compound such as benzoic acid or methoxypolyalkylene glycol. Further, the polyethylene-2,6-naphthalenedicarboxylate may be a copolyester obtained by copolymerization using a trifunctional or higher ester-forming compound such as glycerin or pentaerythritol in such a very small amount that a substantially linear polymer can be obtained.
The polyethylene-2,6-naphthalenedicarboxylate in the present invention may be a composition containing a small amount of other organic polymer.
The above xe2x80x9cother organic polymerxe2x80x9d includes, for example, polyethylene terephthalate, polyethylene isophthalate, polytrimethylene terephthalate, polyethylene-4,4xe2x80x2-tetramethylenediphenyldicarboxylate, polyethylene-2,7-naphthalenedicarboxylate, polytrimethylene-2,6-naphthalenedicarboxylate, polyneopenylene-2,6-naphthalenedicarboxylate and poly(bis(4-ethyleneoxyphenyl)sulfone)-2,6-naphthalenedicarboxylate. Of these, polyethylene isophthalate, polytrimethylene terephthalate, polytrimethylene-2,6-naphthalenedicarboxylate and poly(bis(4-ethyleneoxyphenyl)sulfone)-2,6-naphthalenedicarboxylate are preferred.
The above organic polymers may be contained alone or in combination at least two.
When the organic polymer is incorporated, the amount thereof based on the composition is 10% by weight or less, preferably 5% by weight or less. The above composition may be a composition containing a homopolymer of polyethylene-2,6-naphthalenedicarboxylate and other organic polymer, or it may be a composition containing a copolymer of polyethylene-2,6-naphthalenedicarboxylate and other organic polymer.
Studies made by the present inventors have showed that when polyethylene-2,6-naphthalenedicarboxylate is melted and extruded to form a film, a value of the alternate current volume resistivity of the film has an influence on breaking and productivity of the film in the step of forming the film and also has an influence on the insulating properties of a condenser.
That is, it has been found that a film having an alternate current volume resistivity (Z) of from 5xc3x97107 xcexa9cm or higher to 1xc3x97109 xcexa9cm or less at a temperature of 300xc2x0 C. is preferred. When the value of the alternate current volume resistivity is less than 5xc3x97107 xcexa9cm, the insulating properties of the condenser in a relatively high temperature range of 70xc2x0 C. or higher decrease, the dielectric tangent thereof increases, and the condenser characteristics are degraded. When it exceeds 5xc3x97109 xcexa9cm, the film is frequently broken at the step of forming the film under some film-forming conditions, and the range of conditions of maintaining the stable production of the film is limited.
While the method for obtaining a film having an alternate current volume resistivity in the above range is not specially limited, it is preferred to employ a method of copolymerizing a sulfonic acid quaternary phosphonium salt having an ester-forming functional group into the main chain of the polymer. The content thereof in the polymer chain is preferably at least 0.1 mmol % or more but not more than 10 mmol % based on the total recurring units. Further, the sulfonic acid quaternary phosphonium salt having an ester-forming functional group is preferably a compound represented by the following formula. 
wherein A is an aliphatic group having 2 to 18 carbon atoms or an aromatic group, each having a valence of (n+2), each of X1 and X2 is identical or not and a hydrogen atom or an ester-forming functional group, n is 1 or 2, and each of R1, R2, R3 and R4 is identical or not and an alkyl group having 1 to 18 carbon atoms, benzyl or an aryl group having 6 to 12 carbon atoms, provided that X1 and X2 can be simultaneously hydrogen atoms in no case.
Specific examples of the above sulfonic acid quaternary phosphonium salt preferably include 3,5-dicarboxybenzenesulfonic acid tetrabutylphosphonium salt, 3,5-dicarboxybenzenesulfonic acid ethyltributylphosphonium salt, 3,5-dicarboxybenzenesulfonic acid benzyltributylphosphonium salt, 3,5-dicarboxybenzenesulfonic acid phenyltributylphosphonium salt, 3,5-dicarboxybenzenesulfonic acid tetraphenylphosphonium salt, 3,5-dicarboxybenzenesulfonic acid ethyltriphenylphosphonium salt, 3,5-dicarboxybenzenesulfonic acid butyltriphenylphosphonium salt, 3,5-dicarboxybenzenesulfonic acid benzyltriphenylphosphonium salt, 3,5-dicarbomethoxybenzenesulfonic acid tetrabutylphosphonium salt, 3,5-dicarbomethoxybenzenesulfonic acid ethyltributylphosphonium salt, 3,5-dicarbomethoxybenzenesulfonic acid benzyltributylphosphonium salt, 3,5-dicarbomethoxybenzenesulfonic acid phenyltributylphosphonium salt, 3,5-dicarbomethoxybenzenesulfonic acid tetraphenylphosphonium salt, 3,5-dicarbomethoxybenzenesulfonic acid ethyltriphenylphosphonium salt, 3,5-dicarbomethoxybenzenesulfonic acid butyltriphenylphosphonium salt, 3,5-dicarbomethoxybenzenesulfonic acid benzyltriphenylphosphonium salt, 3-carboxybenzenesulfonic acid tetrabutylphosphonium salt, 3-carboxybenzenesulfonic acid tetraphenylphosphonium slat, 3-carbomethoxybenzenesulfonic acid tetrabutylphosphonium salt, 3-carbomethoxybenzenesulfonic acid tetraphenylphosphonium salt, 3,5-di(xcex2-hydroxyethoxycarbonyl)benzenesulfonic acid tetrabutylphosphonium salt, 3,5-di(xcex2-hydroxyethoxycarbonyl)benzenesulfonic acid -tetraphenylphosphonium salt, 3-(xcex2-hydroxyethoxycarbonyl)benzenesulfonic acid tetrabutylphosphonium salt, 3-(xcex2-hydroxyethoxycarbonyl)benzenesulfonic acid tetraphenylphosphonium salt, 4-hydroxyethoxybenzenesulfonic acid tetrabutylphosphonium salt, bisphenol A-3,3xe2x80x2-di(sulfonic acid tetrabutylphosphonium salt), 2,6-dicarboxynaphthalene-4-sulfonic acid tetrabutylphosphonium salt and xcex1-tetrabutylphosphoniumsulfosuccinic acid. The above sulfonic acid quaternary phosphonium salts may be used alone or two or more in combination.
Generally, the above sulfonic acid quaternary phosphonium salt can be easily prepared by a reaction between a corresponding sulfonic acid and phosphine known per se or a reaction between a corresponding sulfonic acid metal salt and a quaternary phosphonium halide known per se.
In the biaxially oriented film in the present invention, the content of the above sulfonic acid quaternary phosphonium salt in the polyester based on the total recurring units constituting the polyester is at least 0.1 mmol % but not more than 10 mmol %, preferably at least 0.2 mmol % but not more than 10 mmol %. When the above content is less than 0.1 mmol %, it is not possible to attain an alternate current volume resistivity of 109 xcexa9cm or less at 300xc2x0 C. When it exceeds 10 mmol %, the insulating properties may decrease, or the dielectric constant thereof may increase, in a high temperature range (70xc2x0 C. or higher).
In the present invention, the sulfonic acid quaternary phosphonium salt can be incorporated into the film by any method. For example, there may be employed a method in which the above quaternary phosphonium salt compound is added and copolymerized during the polymerization of the polyester (PEN), a method in which the above quaternary phosphonium salt compound is added to the polyester before the formation of the film, or a method in which a polymer composition (master chip) containing a high concentration of the above quaternary phosphonium salt compound is prepared and a predetermined amount thereof is mixed with the polyester as a main raw material before the polyester is melted. In any method, the effect of the sulfonic acid quaternary phosphonium salt is exhibited so long as the polyester (PEN) is allowed to contain a predetermined amount of the sulfonic acid quaternary phosphonium salt.
The method for producing the polyethylene-2,6-naphthalenedicarboxylate can be carried out according to a method of producing a polyester generally known per se. For example, the polyethylene-2,6-naphthalenedicarboxylate can be produced by reacting 2,6-naphthalenedicarboxylic acid with ethylene glycol (esterification), or reacting a lower alkyl ester of 2,6-naphthalenedicarboxylic acid with ethylene glycol in the presence of an ester interchange catalyst (ester interchange reaction), and then polymerizing a reaction product in the presence of a polymerization catalyst up to a desired molecular weight. As the above ester interchange catalyst, a known ester interchange catalyst such as a compound containing sodium, potassium, magnesium, calcium, zinc, strontium, titanium, zirconium, manganese or cobalt can be used, and these compounds may be used alone or two or more in combination. The polymerization catalyst can be preferably selected from antimony compounds such as antimony trioxide and antimony pentoxide; germanium compounds typified by germanium dioxide; and titanium compounds such as tetraethyl titanate, tetrapropyl titanate, tetraphenyl titanate or partial hydrolyzation products of these, titanylammonium oxalate, titanylpotassium oxalate and titaniumtrisacetylacetonate. The amount of each of the catalysts can be selected from conventionally known amount range.
When the polymerization is carried out via the ester interchange reaction, preferably, a phosphorus compound such as trimethylphosphate, triethylphosphate, tri-n-butylphosphoate or orthophosphoric acid is added for deactivating the ester interchange catalyst before the polymerization. The content of a phosphorus element in the polyethylene-2,6-naphthalenedicarboxylate is preferably 20 to 100 ppm in view of thermal stability of the polyester.
Polyethylene-2,6-naphthalenedicarboxylate obtained by melt polymerization may be formed into chips and the chips may be solid-phase-polymerized under heat and reduced pressure or in an inert gas flow.
The intrinsic viscosity (o-chlorophenol, 25xc2x0 C.) of the polyester is preferably at least 0.40 dl/g, more preferably at least 0.40 dl/g but not more than 0.90 dl/g, particularly preferably at least 0.43 dl/g but not more than 0.80 dl/g. The above intrinsic viscosity is the most preferably at least 0.45 dl/g but not more than 0.70 dl/g. When the above intrinsic viscosity is less than 0.40 dl/g, the film may break frequently during the step of forming the film. When it is higher than 0.90 dl/g, undesirably, the melt viscosity of the polyester is high so that it is difficult to melt-extrude it, which results in a long time period of polymerization and a decrease in productivity.
The base film of the present invention is obtained by forming the above polyethylene-2,6-naphthalenedicarboxylate as a main polyester component into a film. The conditions and means of the above film formation will be discussed in detail later. Further, inert particles are incorporated into the polyester for forming the film, and preferred properties of the inert particles will be also discussed in detail later. The surface properties and physical properties of the base film for a condenser in the present invention will be explained below.
In the surface of the base film of the present invention, the central line average roughness (Ra) thereof is 40 to 80 nm, more preferably 45 to 80 nm, particularly preferably 50 to 77 nm. When the central line average roughness (Ra) of the base film surface is less than 40 nm, the film is poor in slipperiness and air escape, and it is difficult to take up the film in a roll. For attaining a central line average roughness (Ra) of over 80 nm, it is required to add particles having a large particle diameter or increase the amount of the particles. However, the film has a small thickness, and the frequency of breaking during the formation of the film therefore increases, so that the productivity extremely decreases.
Further, advantageously, the base film surface has a 10-point average roughness (Rz) of 1,000 to 1,800 nm, more preferably 1,100 to 1,700 nm, particularly preferably 1,200 to 1,700 nm.
When the 10-point average roughness (Rz) is lower than 1,000 nm, the film is poor in slipperiness and air escape, and it is difficult to take up uniformly the film in a roll. For attaining a 10-point average roughness (Rz) of over 1,800 nm, it is required to add particles having a large particle diameter or increase the amount of the particles. However, the film has a small thickness, and the frequency of breaking during the formation of the film therefore increases, so that the productivity extremely decreases.
The base film of the present invention is required to achieve that the number of flyspecks having a certain size in the surface thereof is small.
That is, in the film surface, the number of flyspecks having an average diameter of over 60 xcexcm is 20/m2 or less, more preferably 15/m2 or less, particularly preferably 10/m2 or less. The term xe2x80x9cflyspeckxe2x80x9d refers to a portion which is generated from an additive (lubricant, etc.) or a foreign matter contained in a polymer as a core and which is formed of a small-thickness portion (void) of the core and a polymer forming a film around the core. When the flyspeck is large in size, a region having a small film thickness is broad, so that the film being formed breaks and the insulating performance decreasing due to a concentrated stress when in the step of processing the film into a condenser a tension sharply changes or the film is rapidly heated during the formation of the film or slitting of the film to a proper width. Even if the film does not break, the insulating performance of the film tends to decrease since the film thickness is small. Further, a flyspeck having an average diameter (average of major and minor diameters) of over 60 xcexcm extremely decreases the insulating performance, and when the frequency of presences thereof exceeds 20/m2, the insulating performance as a dielectric for a condenser is insufficient.
For bringing the number of flyspecks having a large size into the above range, it is essential to decrease the number of coarse particles in the polymer to make it as small as possible. For this purpose, preferably, as a filter for use during the formation of the film, there is used a non-woven fabric filter which is made of stainless steel fine wires having a wire diameter of 15 xcexcm or less and which has an average opening of 10 to 30 xcexcm, preferably 15 to 25 xcexcm, and the polymer is filtered. When the above opening of the filter exceeds 30 xcexcm, there is produced no effect on decreasing the coarse particles in a molten polymer. When the opening is less than 10 xcexcm, the filter is liable to be clogged, and practical use thereof is industrially difficult. While other filter having a net-shaped structure or other filter made of a sintered metal, etc., may be available, these filters have problems that they have a short lifetime or are poor in filtering efficiency as compared with the non-woven fabric filter. For more effectively controlling the number of flyspecks having a large size to bring it into the above range, preferably, a lubricant itself is filtered through a filter having a predetermined opening and then added to the polymer.
As is accordingly expected, it is desirable that the number of coarse particles in the base film of the present invention should be small. Specifically, the number of coarse particles having a maximum diameter of over 35 xcexcm per m2 of a film area is desirably 10 or less, preferably advantageously 8 or less, particularly desirably 5 or less.
The above number of coarse particles having a maximum diameter of over 35 xcexcm can be counted on the basis of observation of the film surface through a universal projector as will be described later.
After heat-treated at 200xc2x0 C. for 10 minutes, the base film of the present invention has a thermal shrinkage factor in the longitudinal direction (MD) and a thermal shrinkage factor in the transverse direction (TD) in a constant thermal shrinkage factor ratio (MD/TD).
That is, the thermal shrinkage factor ratio (MD/TD) of the thermal shrinkage factor in the longitudinal direction (MD) and the thermal shrinkage factor in the transverse direction (TD) is 0.39 to 0.82, preferably 0.41 to 0.80, particularly preferably 0.42 to 0.78. When the thermal shrinkage factor ratio in the longitudinal direction and the transverse direction after heat treatment at 200xc2x0 C. for 10 minutes is outside the above range, undesirably, there is caused a problem that heat deterioration takes place during deposition in the step of processing the film into a condenser or that the film after deposition is liable to have wrinkles. In view of processing suitability, after the heat treatment at 200xc2x0 C. for 10 minutes, the heat shrinkage factor in the longitudinal direction (MD) is 0.9 to 3%, preferably 1.0 to 2.8%, and the counterpart in the transverse direction (TD) is 2.0 to 5.0%, preferably 2.2 to 4.8%.
The thickness of the base film of the present invention is preferably 0.5 to 7.0 xcexcm, more preferably 0.6 to 6.0 xcexcm, particularly preferably 0.7 to 5.5 xcexcm. When the thickness of the film is less than 0.5 xcexcm, the film is frequently breaks during the formation of the film and the film formation is difficult since the film thickness is too small. When the thickness of the film exceeds 7.0 xcexcm, the electric properties of the film for a condenser are unsatisfactory.
The thickness of the base film is preferably as uniform as possible. The thickness variability of the film is tolerable so long as it is not more than 25% based on the thickness of the film. The thickness variability based on the film thickness is preferably 20% or less, particularly preferably 15% or less. If the thickness variability of the film based on the film thickness exceeds 25%, undesirably, the thickness variability causes a variability in the performance of a condenser when a laminate of many sheets of such a film is used as a base film for a dielectric of a condenser.
The thickness of the film and the thickness variability of the film are determined on the basis of a method and a definition to be described later.
The base film of the present invention preferably has a density of 1.338 to 1.361 g/cm3. The above density is more preferably 1.340 to 1.358 g/cm3, particularly preferably 1.343 to 1.356 g/cm3. When the above density is less than 1.338 g/cm3, undesirably, a variability is caused on the performance of a condenser, and the processing yield is caused to decrease. When it exceeds 1.361 g/cm3, the crystallizability of the film is too high, and the tenacity of the film disappears, so that the frequency of breaking during the transportation of the film comes to increase.
The method for producing the base film of the present invention will be explained below.
The base film of the present invention is a biaxially oriented film made of polyethylene-2,6-naphthalenedicarboxylate as a main polymer component. This biaxially oriented film can be produced by a usual method, for example, in which the above polymer is melted at a temperature equivalent to, or higher than, the melting point thereof, the melt is extruded onto a cooled casting drum through a die slit, an extruded film is intimately contacted to, cooled on and solidified on the casting drum, to obtain an unstretched film made of polyethylene-2,6-naphthalenedicarboxylate as a main polymer component, the unstretched film is biaxially oriented in the longitudinal and transverse directions, and the stretched film is thermoset and then, optionally, relaxed in the longitudinal direction and/or the transverse direction. The film can be stretched with a known roll-applied longitudinal direction stretching machine, an infrared-heater applied longitudinal direction stretching machine, tenter clip-applied transverse-direction stretching machine, multi-stage stretching machine for carrying out these stretching procedures at a plurality of steps, tubular stretching machine, oven-applied longitudinal direction stretching machine, simultaneous biaxial tenter stretching machine or the like, although the stretching of the film is not specially limited.
The method for producing the base film of the present invention will be explained further in detail below, although it shall not be necessarily limited thereto. The unstretched film obtained in the above manner is stretched 3.0 to 4.5 times, more preferably 3.3 to 4.0 times in the longitudinal direction at 120 to 180xc2x0 C., more preferably 125 to 170xc2x0 C., particularly preferably 130 to 160xc2x0 C. with a roll-applied longitudinal direction stretching machine. While an infrared heater-applied longitudinal direction stretching machine may be used, a roll-applied longitudinal direction stretching machine is preferred when a thin film in particular is stretched since it is advantageous for uniformly heating the entirety of the film. For stretching the film in the longitudinal direction without overstretching, it is preferred to carry out the multi-stage stretching at a plurality of separate steps. After the stretching in the longitudinal direction, in a stenter, the film is further stretched 3.0 to 4.5 times, more preferably 3.5 to 4.3 times in the transverse direction at 120 to 180xc2x0 C., more preferably 125 to 170xc2x0 C., particularly preferably 130 to 160xc2x0 C., and the stretched film is heat-treated at 195 to 250xc2x0 C., more preferably 205 to 245xc2x0 C. for 0.3 to 50 seconds and then thermally relaxed at a relation ratio in the range of from 0.5 to 15% in the longitudinal direction and/or the transverse direction, whereby the desired base film can be obtained. For the stretching in the transverse direction, multi-stage stretching at a plurality of steps may be employed.
Inert particles are incorporated into the polymer so that the base film of the present invention has the above surface properties and exhibits the above physical properties. The inert particles added therefor are preferably a combination of two types of particles having different forms. Further, an amount of the inert particles also has an influence on the surface properties.
According to studies made by the present inventors, it has been found that the form, kind and amount of the inert particles to be added to the polymer are advantageously selected from particle incorporation modes (P-1) to (P-4) to be explained below.
(1) Particle Incorporation Mode (P-1)
This mode (P-1) is to incorporate a composition prepared by combining 0.03 to 1.5% by weight of spherical silica particles (A) having an average particle diameter of 0.5 to 3.0 xcexcm and 0.05 to 2% by weight of spherical silica particles (B) having an average particle diameter of 0.01 to 1.5 xcexcm. For stably producing the film, further, the average particle diameter of the spherical silica particles is required to be smaller than the intended film thickness, and the above average particle diameter is preferably 90% or less of the film thickness, particularly preferably 80% or less of the film thickness. The average particle diameter of the spherical silica particles (A) is more preferably 0.7 to 2.5 xcexcm, particularly preferably 0.9 to 2.0 xcexcm, and the amount of the spherical silica particles (A) is more preferably 0.06 to 1.2% by weight, particularly preferably 0.1 to 0.8% by weight.
Further, the average particle diameter of the spherical silica particles (B) is more preferably 0.05 to 1.2 xcexcm, particularly preferably 0.1 to 1.0 xcexcm. The amount of the spherical silica particles (B) is more preferably 0.08 to 1.5% by weight, particularly preferably 0.1 to 1.0% by weight.
As the spherical silica particles (A), spherical silica particles having a larger average particle diameter than the spherical silica particles (B) are used, and preferably, the average particle diameter of the spherical silica particles (A) is larger by 0.1 xcexcm or more.
The above spherical silica particles are preferably spherical silica particles having an aspect ratio (major diameter/minor diameter) of 1.0 to 1.2, and in view of film formability, handling properties and insulating properties, truly spherical silica particles are particularly preferred.
(2) Particle Incorporation Mode (P-2)
This mode (P-2) is the following porous silica particles (A) or a combination of the particles (A) with the following spherical silica (B).
The porous silica particles (A) are preferably formed of aggregates of primary particles having an average particle diameter of 0.001 to 0.1 xcexcm. While the porous silica particles (A) exhibit high affinity to polyethylene-2,6-naphthalenedicarboxylate, they often contain coarse particles since they are formed of aggregates. When the coarse particles are contained in the film, the performance of the polyester film for a condenser is caused to decrease. When the average particle diameter of the primary particles is less than 0.001 xcexcm, undesirably, the particles come to easily aggregate since they have a large surface area, so that coarse aggregates are formed. When the average particle diameter of the primary particles exceeds 0.1 xcexcm, undesirably, the particles come to lose their porosity, their affinity with polyethylene-2,6-naphthalenedicarboxylate is lost, a void is liable to occur around a lubricant, and the insulating properties are degraded. In view of the slipperiness and air escape of the film, the average particle diameter of the porous silica particles (A) is preferably 0.5 to 5 xcexcm, more preferably 0.7 to 4.0 xcexcm, particularly preferably 1.0 to 3.0 xcexcm. The amount of the porous silica particles (A) is preferably 0.05 to 2% by weight, more preferably 0.07 to 1.8% by weight, particularly preferably 0.1 to 1.5% by weight.
The pore volume of the porous silica particle (A) is preferably 0.5 to 2.0 ml/g, more preferably 0.6 to 1.8 ml/g. When the above pore volume is less than 0.5 ml/g, undesirably, the porous silica particle (A) is poor in porosity and the affinity with polyethylene-2,6-naphthalenedicarboxylate no longer exists. When the pore volume exceeds 2.0 ml/g, undesirably, aggregation is liable to take place, and it is difficult to adjust the particle diameter.
In the above mode (P-2), the above porous silica particles (A) are used. In the extrusion step or recovery step in the production of the film, the porous silica particles (A) are milled to decrease their particle diameters, which involves the deterioration of slipperiness. For preventing the deterioration of slipperiness, spherical silica particles (B) may be added in combination.
The spherical silica particles (B) to be added preferably have an average particle diameter of 0.05 to 1.5 xcexcm, and for producing the film stably, the average particle diameter is required to be smaller than the film thickness. The average particle diameter is preferably 90% or less of the film thickness, and particularly preferably 80% or less of the film thickness. In view of slipperiness and take-up properties of the film and handling properties in the step of producing a condenser, the amount of the spherical silica particles (B) is preferably 0.01 to 1% by weight, more preferably 0.03 to 0.9% by weight. Further, the aspect ratio (major diameter/minor diameter) of the spherical silica particles (B) is preferably 1.0 to 1.2.
In this mode (P-2), particularly preferred is a combination of the porous silica particles (A) and the spherical silica particles (B).
(3) Particle Incorporation Mode (P-3)
This mode (P-3) is the following calcium carbonate particles (A) or a combination of the calcium carbonate particles (A) with the following plate-like aluminum silicate particles (B).
In view of slipperiness and air escape of the film, the average particle diameter of the calcium carbonate particles (A) is preferably 0.2 to 5 xcexcm, more preferably 0.3 to 4 xcexcm, particularly preferably 0.5 to 3 xcexcm. Further, the amount of the calcium carbonate particles (A) is preferably 0.03 to 2% by weight, more preferably 0.05 to 1.5% by weight, particularly preferably 0.1 to 1% by weight.
Although not specially limited, the calcium carbonate particles (A) used in the present invention include naturally occurring calcite crystals such as such as limestone, chalk (whiting) and precipitated calcium carbonate formed from lime stone by a chemical method, aragonite obtained by reacting milk of lime with carbon dioxide gas at high temperature, vaterite crystal and any combination of these. Heavy calcium carbonate (calcite crystal) obtained by mehcanically milling limestone may be also used.
While the above mode (P-3) uses the above calcium carbonate particles (A), plate-like aluminum silicate particles (B) may be used in combination.
The average particle diameter of the above aluminum silicate particles (B) is preferably 0.1 to 2 xcexcm, more preferably 0.3 to 1.7 xcexcm, particularly preferably 0.5 to 1.5 xcexcm. In view of slipperiness of the film and handling properties in the step of producing a condenser, the amount thereof is preferably 0.03 to 1% by weight, more preferably 0.06 to 0.8% by weight, particularly preferably 0.1 to 0.7% by weight.
The plate-like aluminum silicate particles (B) refer to aluminosilicate, and are not specially limited. Examples thereof include kaolin clay formed from naturally occurring kaolin mineral, and the like. Further, the kaolin clay may be a product obtained by purification such as washing.
In the above mode (P-3), a combination of the calcium carbonate particles (A) with the aluminum silicate particles (B) is particularly preferred.
(4) Particle Incorporation Mode (P-4)
This mode (P-4) is a combination of two types of porous silica particles (A) and (B). Each type of the porous silica particles (A) and (B) are particles composed of aggregates of primary particles having an average particle diameter of 0.01 to 0.1 xcexcm.
The porous silica particles shows high affinity to polyethylene-2,6-naphthalenedicarboxylate (PEN), and often contain coarse particles since they are formed of aggregates. When the coarse particles are contained in the film, the coarse particle cause the performance of the polyester film for a condenser to decrease. When the average particle diameter of the primary particles is less than 0.01 xcexcm, undesirably, the particles come to easily aggregate since they have a large surface area, so that coarse aggregates are formed. When the average particle diameter of the primary particles exceeds 0.1 xcexcm, undesirably, the particles come to lose their porosity, their affinity with PEN is lost, a void is liable to occur around a lubricant, and the insulating properties are degraded.
In view of slipperiness and air escape of the film, the average particle diameter of the porous silica particles (A) in the mode (P-4) is at least 0.1 xcexcm but not more than 1.5 xcexcm, particularly preferably at least 0.3 xcexcm but not more than 0.9 xcexcm. The porous silica particles (A) work to finely roughen a ground between relatively large protrusions formed of porous silica particles (B). When the average particle diameter of the particles (A) is less than 0.1 xcexcm, the effect of roughening the ground is small, and it is difficult to secure the slipperiness. When it exceeds 1.5 xcexcm, undesirably, a space factor is too large, and a volume is liable to be deficient.
The average particle diameter of the porous silica particles (B) is at least 0.7 xcexcm but not more than 5.0 xcexcm, particularly preferably at least 1.1 xcexcm but not more than 3.0 xcexcm. When the above average particle diameter is less than 0.7 xcexcm, the air escape is insufficient, and the take-up form of the film is poor. When it exceeds 5.0 xcexcm. undesirably, coarse particles increase in number, many insulation defects occur, and a space factor is too large. As the porous silica particles (B), porous silica particles having a larger average particle diameter than the porous silica particles (A) are used, and it is preferred to use the porous silica particles (B) having an average particle diameter larger than that of the porous silica particles (A) by 0.1 xcexcm or more.
The amount of each type of the porous silica particles (A) and (B) is at least 0.01% by weight but not more than 2.0% by weight, and the total amount of the porous silica particles (A) and (B) is at least 0.1% by weight but not more than 2.0% by weight, particularly preferably at least 0.5% by weight but not more than 1.5% by weight. When the above amount is less than the lower limit of the above range, no lubricating effect is exhibited and the workability is too poor to use the film. When the above amount exceeds the upper limit of the above range, undesirably, a space factor is too large.
Each type of the porous silica particles (A) and (B) have a pore volume of at least 0.5 ml/g but not more than 2.0 ml/g, and the above pore volume is preferably at least 0.6 ml/g but not more than 1.8 ml/g. When the pore volume is less than 0.5 ml/g, undesirably, the porosity thereof is poor, and the affinity with PEN no longer exists. When the pore volume exceeds 2.0 ml/g, undesirably, aggregation is liable to take place, so that it is difficult to adjust their particle diameters.
The above term xe2x80x9caverage particle diameterxe2x80x9d means an xe2x80x9cequivalent sphere diameterxe2x80x9d of a particle existing at a 50% by weight of all the particles measured. The xe2x80x9cequivalent sphere diameterxe2x80x9d means a diameter of an imaginary sphere (ideal sphere) having the same volume as that of a particle and can be calculated on the basis of an electron microscopic photograph of particles or measurement by a general precipitation method.
When the above porous silica particles (A) and (B) are added, it is preferred to adjust them such that the number of coarse particles having a major diameter of over 35 xcexcm in the film is 10/m2 or less, more preferably 8/m2 or less, particularly preferably 5/m2 or less. When the above number exceeds 10/m2, undesirably, breaking of the film frequently occurs during the formation of the film, which causes the productivity to decrease, and many insulation defects of a condenser are caused.
For bringing the number of the coarse particles into the above range, the polymer is filtered during the formation of the film to remove the coarse particles. As a filter therefor, it is preferred to use a non-woven fabric filter which is made of stainless steel fine wires having a wire diameter of 15 xcexcm or less and which has an opening average of 10 to 35 xcexcm.
The base film of the present invention obtained by the above method is formed into a composite film for a condenser by laminating an electrically conductive metal thin layer on one surface or both the surfaces thereof. For laminating the electrically conductive metal thin layer on the base film surface, a deposition method and a sputtering method are preferred, and a deposition method is particularly preferred.
The thickness of the metal thin layer is 0.01 to 0.1 xcexcm, preferably 0.03 to 0.08 xcexcm. The metal is selected from aluminum, zinc or tin, and aluminum is particularly preferred.
The composite film for a film condenser, provided by the present invention, constitutes a high-quality condenser member whose insulation defects are very small in number since the base film thereof is excellent in surface properties and physical properties.
That is, the composite film of the present invention constitutes a film in which the number of portions failing to satisfy a dielectric breakdown voltage of 200 V/xcexcm (electrical insulation defects) is 20/m2 or less, preferably 15/m2 or less.
Further, the composite film of the present invention has an excellent CR value that shows characteristics as a film condenser member. This CR value is expressed by a product of an insulation resistance value and an electrostatic capacity as will be discussed later. The composite film of the present invention has a CR value of 800 xcexa9F or more, preferably 1,000 xcexa9F or more. When the above CR value is less than 800 xcexa9F, the insulation resistance of the composite film is insufficient, and the composite film is unsuitable as a film condenser member.
The composite film of the present invention is used as a film condenser member. For example, the composite film may be used in the form of a wound sheet of the film or a laminate of sheets of the film. Further, it can be used as chip condenser. In any case, the composite film of the present invention has excellent insulating properties and has a very small thickness, so that it is used as a small-sized film condenser member.